This invention relates generally to internal combustion engines and more specifically an internal combustion engine used to power an aircraft.
As is known in the art, a considerable amount of attention has been given to the elimination of engine knock, or spark knock, that is produced during the combustion process in an internal combustion engine. Spark knock refers to the sound and related effects produced in spark ignited internal combustion engines by instantaneous ignition and combustion, i.e., autoignition, of the gaseous fuel-air mixture ahead of the advancing flame front. After spark ignition, the flame front travels outward from the spark plug and, under normal combustion, will progressively burn the entire fuel-air charge. The burned gas liberates heat and expands, leading to increased pressure and temperature in the unburned gas ahead of the flame front. In addition, the cylinder pressure and temperature increase due to the upward, compressive motion of the piston. The result is that the unburned fuel may be raised above its autoignition temperature. However, before autoignition occurs, time must be allowed for the chemical reactions that precede knock. If the flame front passes through the unburned gas before these reactions are completed, normal combustion results. If these reactions occur too quickly or if the flame front velocity is too small, the unburned gas spontaneously ignites and burns instantaneously. This process is also referred to as detonation. The instantaneous combustion results in a very intense pressure wave that is internally reflected within the cylinder at a characteristic frequency related to the velocity of sound within the cylinder and the dimensions of the combustion chamber. The flexing of the cylinder wall and cylinder head produces the audible, high-frequency pinging sound known as spark knock. Besides sound, spark knock leads to structural damage of the combustion chamber and engine and loss of efficiency.
A popular way to reduce knock in an automobile internal combustion engine is by increasing the octane rating of the gasoline, where the octane rating is a measure of the fuel""s resistance to knock. The higher the octane rating, the more resistant the fuel is to knocking. Higher octane numbers are due to higher autoignition temperatures or longer end-gas chemical reaction times. Either fuel structure or fuel additives determine octane rating. More compact hydrocarbon molecules have higher octane numbers than do long-chain molecules. For many years, the most popular anti-knock additive was lead. However, because of the elimination of leaded gas for automobile use in the United States and because other anti-knock additives are not as effective as lead, manufactures of automobile engines turned to improved combustion chamber design to prevent knock.
As is known in the automotive industry, combustion chamber designs that increase temperature, pressure, and chemical resistance time of the unburned gas (end gas) increase spark knock. Increased compression ratio, off-center plug location, and slow-burn combustion chambers also lead to increased spark knock. Conversely, a faster-burning chamber, due to higher in-cylinder gas velocity and turbulence, and central plug location increase knock resistance. Faster-burning chambers are helpful in eliminating knock because the last part of the charge is burned by the flame front before it has time to spontaneously ignite, i.e., detonate. Characteristics of faster-burning chambers include the use of high swirl intake ports and a rotational motion (swirl) of the charge due to off-cylinder axis charge admission); the use of two or more spark plugs; and inducement of small-scale turbulence in the cylinder charge achieved by designing the chamber so that part of the piston head comes close to the cylinder head at top dead center to thereby xe2x80x9csquishxe2x80x9d the charge in this region into the rest of the combustion chamber and toward the spark plug tips. Another way to produce a faster-burning chamber to reduce knock is by fuel enrichment, i.e., increasing the air-fuel mixture ratio. This also helps to cool the engine. However, the penalty associated with fuel enrichment is reduced fuel economy.
Heretofore, the advancements in combustion cylinder design directed at reducing spark knock described above have not occurred in piston-cylinder engines used in aircraft. One reason for this is that the Federal government has not yet phased out the use of lead as an additive for aviation gasoline. Thus, the use of high octane, leaded gasoline has been the primary method to reduce knock in piston-cylinder aircraft engines. However, because of continuing environmental concerns, the future availability of high octane aviation gasoline is in doubt.
Therefore, there is a need to provide an internal combustion chamber design for an aircraft engine that significantly reduces knock without the need to burn leaded aviation gasoline.
Accordingly, the present invention provides an improved, internal combustion aircraft engine, the internal combustion aircraft engine comprising a cylinder head having a cylindrical head bore formed therein, a piston, and spark plugs. The cylinder head is fixed attached to a top end of a cylinder barrel formed with cylindrical walls, the cylinder head bore and the cylindrical walls defining a cylinder. The cylinder head bore has formed therein an outwardly protruding cavity with respect to the top end of the cylinder barrel. The piston has a piston crown, and the piston contained within the cylinder, the piston crown, the cylinder walls, and the cylinder head bore define a combustion chamber. The piston crown has a concavity formed therein, and the piston crown and the cavity cooperate at a top dead center position of the piston to form a swirl chamber. The piston crown has formed along a peripheral edge thereof a second squish area that corresponds to a second squish area formed in the cylinder head bore, so that when the piston approaches top dead center, the squish area and the second squish area cooperate to cause combustion gases to move radically inward of the combustion chamber and into the swirl chamber. Spark plugs extend through the cylinder head, the tips of which are disposed within the swirl chamber.
Other features and characteristics of the present invention, as well as the methods of operation of the invention and the function and interrelation of the elements of structure will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this disclosure, wherein like reference numerals designate corresponding parts in the various figures.